CHARACTERIZATION OF HIGH-TEMPERATURE OXIDE-FILMS ON STAINLESS-STEELS BY ELECTROCHEMICAL-IMPEDANCE SPECTROSCOPY

Oxide films formed on three stainless steels (UNS S30403; S44600; S308 15) in air at 800 degrees C were characterized by electrochemical-impe dance spectroscopy (EIS). The film evolution vs, oxidation time was in vestigated from 1 to 1000 hr. A three-electrode electrochemical cell a nd 0.1 M Na2SO4 solution were employed for EIS measurements. The spect ra were interpreted in terms of a two-layer model of the films, where the capacitance and resistance obtained can be related to the thicknes s (or roughness) and defectiveness of the films. The results reveal th at the oxide on S30403 grows and becomes defective, the oxide on S4460 0 thickens rapidly and retains its protective ability for a relatively long time, and the oxide on S30815 remains thin and resistive. The tw o-layer model is supported by surface characterization with SEM/EDS an d in-depth profile of the oxide films obtained through glow discharge optical emission spectroscopy (GDOES).